Method for removing at least one area of a layer of a component consisting of metal or a metal compound

ABSTRACT

The invention relates to a method for removing an area of a layer of a component consisting of metal or a metal compound. According to prior art, corrosion products of a component are removed in a first step by applying a molten mass or by heating in a voluminous powder bed. This requires high temperatures or a large amount of space. The inventive method for removing corrosion products of a component is characterized in that a cleaning agent is applied locally, which removes the corrosion products by means of a gaseous reaction product.

FIELD OF THE INVENTION

[0001] The invention relates to a method for removal of a layer area ofa part composed of metal or of a metal compound, in which amulticomponent cleaning agent is applied in a simple manner to the partor to the layer area, as a result of which, after heat treatment of thepart with the cleaning agent, the layer area can be removed more easily.

BACKGROUND TO THE INVENTION

[0002] In present-day modern power generating systems, such as gasturbine systems, the efficiency plays an important role, because thismakes it possible to reduce the costs for operation of the gas turbinesystems.

[0003] One possible way to improve the efficiency and thus to reduce theoperating costs is to increase the inlet temperatures of a combustiongas within a gas turbine.

[0004] Ceramic heat insulation layers have been developed for thisreason, which are applied to thermally loaded parts which, for example,are composed of superalloys, which on their own could no longerwithstand the high inlet temperatures in the long term. The ceramic heatinsulation layer offers the advantage of good temperature resistanceowing to its ceramic characteristics, and the metallic substrate offersthe advantage of good mechanical characteristics in this composite orlayer system.

[0005] Typically, an adhesion promotion layer composed of MCrAlY (majorparts) is applied between the substrate and the ceramic heat insulationlayer, with M indicating that a metal composed of nickel, chromium oriron is used.

[0006] The composition of these MCrAlY layers may vary, but all theMCrAlY layers are subject to corrosion, despite the ceramic layer onthem, due to oxidation, sulfidation, nitridation or other chemicaland/or mechanical attacks.

[0007] The MCrAlY layer in this case is frequently degraded to a greaterextent than the metallic substrate, that is to say the life of thecomposite system comprising the substrate and layer is governed by thelife of the MCrAlY layer.

[0008] The MCrAlY intermediate layer is still functional only to arestricted extent after lengthy use while, in contrast, the substratemay still be fully functional.

[0009] There is therefore a requirement to reprocess the parts whichhave become degraded in use, for example turbine blades, guide vanes orcombustion chamber parts, in which process the corroded layers or zonesof the MCrAlY layer must be removed, in order, possibly, to apply newMCrAlY layers and/or a heat insulation layer once again. The use ofexisting, used substrates leads to a cost reduction during operation ofgas turbine systems.

[0010] In this case, care must be taken to ensure that the design of theturbine blades or of the guide vanes is not changed, that is to say thatthe material is removed from the surface uniformly.

[0011] Furthermore, no corrosion products must be left behind whichwould form a fault source when a MCrAlY layer and/or a ceramic heatinsulation layer is coated once again, or which would lead to pooradhesion of the heat insulation layer.

[0012] A method for removal of corrosion products is known from U.S.Pat. No. 6,217,668. In this method, the corroded part is accommodated ina large vat, with the part being arranged in a powder bed with analuminum source. The vat must be partially closed and then heated in anoven. The heating process results in aluminum being supplied to thecorroded part, as a result of which the areas can be removed by means ofa subsequent acid treatment which would previously not have been able toremove it as well, that is to say it would have had greater resistanceto removal.

[0013] A large amount of material is required for the powder bed, andthe vat occupies a large amount of space in the oven during the heattreatment. The heating process also takes longer, owing to the high heatcapacity.

[0014] A further method for removal of surface layers from metalliccoatings is known from U.S. Pat. No. 6,036,995. In this method, thealuminum source is applied by means of a paste to a corroded part.However, the part must be heated with the paste until the aluminummelts, so that the aluminum does not diffuse into the part until thisstage. The melted aluminum layer is difficult to remove, since itadheres to the part very well.

DESCRIPTION OF THE INVENTION

[0015] The invention overcomes the described disadvantage by means of amethod as described in claim 1.

[0016] In contrast, the method according to the invention has theadvantage that layer areas and/or corrosion products can be removed fromparts in a simple manner. This for the first time makes it possible tocarry out the deposition of an impregnation agent from the gas phase ina locally controllable method, so that no impregnation takes place inareas which are intended to remain untreated, despite the gaseousbonding with the impregnation agent.

[0017] The method steps which are described in the dependent claimsallow advantageous developments and improvements of the method specifiedin claim 1.

[0018] It is advantageous to at least roughly remove the corrosionproducts or other areas, such as a heat insulation layer on a turbineblade, in an intermediate step of the method according to the inventionbefore the application of a cleaning agent to the part or the layerarea, because this simplifies the subsequent method steps, shortens thetime involved, and thus reduces the costs.

[0019] The removal process can be carried out by mechanical methods, forexample sandblasting, water jets, dry ice jets, and/or by chemicalmethods, for example an acid treatment.

[0020] If the cleaning agent at least partially adheres to the part,then, for example, corrosion products can be removed from the front faceand rear face of the part at the same time, using the method accordingto the invention, in an advantageous manner.

[0021] The adhesion of the cleaning agent to the part can advantageouslybe carried out by the cleaning agent having a pasty consistency by, forexample, the cleaning agent containing a binding agent.

[0022] The cleaning agent can also be mixed with a carrier liquid withor without a binding agent and can be brushed onto the part, or the partcan be coated with the cleaning agent by immersion in a compound whichcan flow and is composed of liquid and cleaning agent. The cleaningagent may also advantageously be applied only locally to the part, sinceareas which are not corroded do not need to have the cleaning agentapplied to them, thus making it possible to save cleaning agent.

[0023] There is therefore no longer any need for masks either, in orderto protect those areas in which no cleaning agent need be applied, aswhen application is carried out over a large area (powder bed, plasmaspraying, running aluminum melt).

[0024] The cleaning agent is advantageously applied in the vicinity ofthe corrosion products because this results in the at least onecomponent of the cleaning agent having only short diffusion distances totravel during the heat treatment.

[0025] By way of example, the cleaning agent is applied in a thin layerto the part, so that considerably less material is used than when thepart is embedded in a powder bed. Furthermore, heat treatment withoutany vat means that no space is consumed by the voluminous vat in theoven, so that more parts can be accommodated in one oven cycle, thusreducing the process costs.

[0026] The lack of and the reduction in the masses of vats and cleaningagents, respectively, means that considerably less mass may be heatedoverall.

[0027] The removal process is carried out uniformly over the surface ofthe uncorroded part, by means of a removal method or an acid treatment.However, the corrosion produces areas on the part and/or corrosionproducts which can no longer be removed as easily by the acid treatment,that is to say they are more resistant to removal. If an acid treatmentis used as the removal method, this leads to undesirable, non-uniformremoval from corroded or degraded parts.

[0028] The formation of at least one sacrificial zone in the layer areato be removed, which is achieved by the treatment according to theinvention, that is to say the areas of the part which are more resistantto removal, means that those areas which have become more resistant toremoval by degradation can be removed in the same way as material on thenon-degraded part, and the high resistance to removal which exists inany case in a layer area which is not degraded is reduced.

[0029] This allows corroded and uncorroded material to be removed fromthe part uniformly.

[0030] In the case of MCrAlY layers, the sacrificial zone advantageouslyhas a metallic impregnation component, advantageously aluminum, analuminum compound or an aluminum alloy.

[0031] The cleaning agent may also advantageously contain the metalcomponent in the form of a metal complex. There is therefore no need,for example, to mix a metallic powder with a carrier substance or withthe activation agent.

[0032] The impregnation component must at least partially diffuse out ofthe cleaning agent into the part. This is advantageously achieved by theimpregnation component being applied to the part in a gaseous form. Thegaseous compound is produced by a reaction with the activation agent,with the impregnation means advantageously not being melted, thusreducing the process temperatures and hence the process costs.

[0033] Halogen compounds, for example ammonium chloride, which formsaluminum chloride with aluminum, are advantageously used as a cheap andeasily available activation agent.

[0034] The formation of the gaseous compound can be controlled byadvantageously mixing a carrier substance, for example aluminum oxide,with the cleaning agent, thus controlling the gas formation process, andmaking it uniform.

[0035] The method is advantageously suitable for layer systems such as aturbine blade, which have a layer system comprising a metallicsubstrate, an MCrAlY layer and a ceramic heat insulation layer appliedto it.

[0036] Corrosion products on the MCrAlY layer lead to depletion ofaluminum in the MCrAlY layer underneath the corrosion products (Al₂O₃)and, in consequence, these are more resistant to acid treatment. If thecleaning agent contains aluminum as a metallic component, the aluminumonce again provides aluminum enrichment, on the basis of the methodaccording to the invention, in those regions of the MCrAlY layer whichwere previously depleted of aluminum, so that these areas can then beresolved in the same way as the MCrAlY layer by means of an acidtreatment, resulting in the corrosion products which are located onthese areas also being dissolved.

[0037] The method according to the invention allows layer areas whichare resistant to removal to be removed in an advantageous manner, orelse degraded areas, for example areas which contain corrosion productswhich form a layer on the corroded part, or else corrosion productswhich are located underneath the surface of the corroded part.

[0038] After a certain heat treatment time, the area of the cleaningagent which is arranged on the part, close to the surface of the part,is depleted of the at least one impregnation component. The heattreatment is thus ended once the sacrificial zones are large enough,that is to say in the case of an MCrAlY layer, once the regions whichwere depleted of aluminum have been sufficiently enriched with aluminumonce again. If this is not yet the case, the cleaning agent can beremoved and the part can then be subjected to a thermal treatment, withthe impregnation component of the cleaning agent, which is alreadypresent in the part as a result of the diffusion process, advantageouslybeing allowed to penetrate deeper by diffusion into the part, thusincreasing the depth of the sacrificial zone or sacrificial layer in anadvantageous manner.

[0039] An optimum temperature for the thermal treatment is higher thanthe temperature for the heat treatment but below the solution annealingtemperature of the part.

[0040] Exemplary embodiments of the method according to the inventionare illustrated in the figures, in which:

[0041]FIG. 1 shows a corroded metallic part,

[0042]FIG. 2 shows a part to which a cleaning paste has been appliedwhich contains a metallic component which penetrates, by virtue of afurther method step, into the corroded area (FIG. 3) and only thenallows the corroded area of the part to be dissolved (FIG. 4),

[0043]FIGS. 5 and 6 show a layer system in which one layer has corrodedareas,

[0044]FIG. 7 shows a layer system,

[0045]FIG. 8 shows degraded areas of a layer in the layer system, whichare removed by means of the method according to the invention (FIG. 9),

[0046]FIG. 10 shows a substrate with a degraded area, which is removedby means of the method according to the invention (FIG. 11), and

[0047]FIG. 12 shows a layer system with a chromium layer, which isremoved by means of the method according to the invention (FIG. 13).

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0048]FIG. 1 shows a part 1 composed of metal, of a metal alloy, or of ametal compound which has external corrosion products 4 on a surface 7and/or has internal corrosion products 5 on the interior of the part 1,which corrosion products are present, for example, in regions which areformed separately from one another. The corrosion products 4 may also becohesive or may be present over the entire surface 7, that is to sayforming a corrosion layer.

[0049] The part 1 may be solid, may be a layer or may be an area of acomposite or layer system 16 (FIGS. 5, 6). The corrosion products 4, 5have been formed during use of the part 1 and are undesirable forfurther use for the part 1, and must be removed. This is frequently doneby treatment in an acid bath.

[0050] However, the material of the part 1 may have degraded areas andthe corrosion products 4, 5 may react differently in the acid bath. Thedifferent dissolving characteristics in the acid bath are caused by thedifferent dissolving characteristics of the corrosion products 4, 5, orbecause an original composition of the material of the part 1 haschanged (FIGS. 5, 6), for example because the corrosion product 4, 5 hasextracted a component from an area of the part 1 in the area around thecorrosion product 4, 5, the so-called depletion region. This results innon-uniform removal or no removal of the corrosion products, or of thematerial in the depletion region.

[0051] The method according to the invention allows the corrosionproducts to be removed completely and uniformly with the material of thepart 1.

[0052] In a first method step, by way of example, the corrosion productsor other areas may in this case be removed by mechanical methods, suchas sandblasting and/or chemical means, such as an acid bath.

[0053] In a further method step, a multicomponent cleaning agent 10 isapplied to the corroded part 1, in particular in the areas with thecorrosion products 4, 5 which, in this example, represent the areaswhich are resistant to removal (FIG. 2), that is to say the layer area52. The layer area 52 to be removed is identified by a dashed line, andcomprises all the corrosion products 4, 5.

[0054] The cleaning agent 10 contains at least one impregnationcomponent 13 which, during heat treatment, reacts with at least oneactivation component of the cleaning agent 10 to form at least onegaseous compound.

[0055] The gaseous compound results in the impregnation component 13being brought into contact with the part 1 or being precipitated therewhere, for example, it forms an impregnation layer in the material ofthe part 1. The impregnation agent diffuses from this impregnation layeror directly from the gaseous compound into the areas with the corrosionproducts 4, 5. The impregnation component 13 is then at least partiallypresent in the areas with the corrosion products 4, 5.

[0056] The area which is formed in this way, the so-called sacrificialzone 25 (FIG. 3), can be removed uniformly together with the material ofthe part 1, for example by means of an acid bath. A layer area 52 to beremoved is identified by a dashed line. The layer area 52 to be removedcomprises all of the corrosion products, but may also be deeper than thedeepest corrosion product 5.

[0057] The acid treatment reduces the thickness of the part 1 from athickness d (FIG. 3) to a lesser thickness d′ (FIG. 4).

[0058]FIG. 4 shows a part 1 without any internal or external corrosionproducts 4, 5, as a result of the treatment based on the methodaccording to the invention.

[0059] The choice of the material for the at least one impregnationcomponent depends on the composition of the material of the part 1and/or of the corrosion products 4, 5.

[0060] The activation component has the object of applying theimpregnation component to the surface 7 of the part. This is achievedbecause the activation component can form a gaseous compound with theimpregnation component, and this gaseous compound can be deposited onthe surface 7 of the part 1. Halogen compounds, for example, may be usedfor this purpose.

[0061] With regard to the method for application of the cleaning agent,reference is made to U.S. Pat. No. 6,217,668, which is expresslyincluded as part of this disclosure.

[0062]FIG. 5 shows a layer system 16 as a part 1, by way of example inthe form a turbine blade or guide vane.

[0063] In this case, the layer system 16 comprises a substrate 19, forexample composed of a superalloy, for example with the basic compositionNi₃Al. A layer 22 is applied to the substrate 19, for example with thecomposition MCrAlY, where M represents a chemical element Cr, Ni or Fe.This so-called MCrAlY layer forms a corrosion protection layer, whichcan also act as an adhesion promotion layer for a ceramic heatinsulation layer which is not illustrated but is applied to the layer22.

[0064] During use of the layer system 16, oxidation, nitridation orsulfidation occur, by way of example, that is to say degradation of theMCrAlY layer 22, so that areas with corrosion products 4, 5 (not shown)are formed in the layer 22.

[0065] The corrosion products 4, 5 form a layer which exists at least inplaces in, on or underneath the surface 7 of the part 16.

[0066] These corrosion products 4, for example aluminum oxide or otheraluminum compounds, extract aluminum from the MCrAlY layer 22, so thatat least one sacrificial zone 25 of aluminum-depleted MCrAlY is formedin the vicinity of the area with the corrosion products 4, mainlyunderneath the corrosion products, that is to say in the direction ofthe substrate 19. These depleted regions in this example represent thearea which is more resistant to removal, that is to say the layer area52. The layer area 52 to be removed is identified by a dashed line, andcomprises all of the corrosion products 4, 5, or the entire layer 22.

[0067] The MCrAlY layer may also be depleted of chromium (Cr), so thatthe impregnation component 13 has, for example, the elements Al and/orCr.

[0068] The impregnation component 13 may also contain other metals, forexample cobalt, or elements or combinations thereof.

[0069] Both the corrosion products 4 and the sacrificial zone 25 havegreater resistance to acid in the acid bath than the material of thelayer 22, that is to say the MCrAlY.

[0070] In a first method step, the ceramic heat insulation layer, thecorrosion products or other areas can be removed roughly by mechanicalmethods, such as sandblasting and/or chemical means, for example an acidbath.

[0071] The application of the cleaning agent 10 with the metal component13 and the subsequent heating results in diffusion of the metalcomponent 13 which, in this example, contains aluminum, both into theareas with the corrosion products 4 and into the sacrificial zones 25,so that the at least one metal component 13 is provided there. After,and only after, the enrichment with the metal component 13, a specificlayer thickness of the layer 22 (MCrAlY) can be removed uniformly inacid bath treatment of the layer system 16.

[0072] The cleaning agent 10 may also have two or more metalliccomponents 13 (Al, Cr) if this is required for the composition of thecorrosion products or of the depleted sacrificial zones 25.

[0073] The metallic component 13 is, for example, mixed with at leastone carrier substance, for example aluminum oxide or aluminum silicate.The cleaning agent 10 may also contain the metallic component 13 in theform of a metal complex.

[0074] The cleaning agent 10 likewise has at least one activation agent,for example a halogen compound, for example in the form of ammoniumchloride (NH₄Cl).

[0075] During the heat treatment of the part 1 with the cleaning agent10, the aluminum reacts as the metal component 13 with the

[0076] halogen compound to form a gaseous compound. With ammoniumchloride as the example, this gaseous compound is aluminum chloride. Thegaseous compound penetrates into the at least one sacrificial zone 25and allows the aluminum to diffuse into the part 1 by, for example,forming an impregnation layer (FIG. 6). There is therefore no need forthe metal component 13 to be melted. However, it is also possible forthe gaseous compound to be formed only at temperatures which are abovethe melting point of the at least one impregnation component since, forexample, sublimation occurs.

[0077] In the example of aluminum fluoride, the impregnation component13 and the activation component are contained in one compound (forexample AlF₃). A gaseous compound aluminum fluoride (AlF) is formedduring the heat treatment.

[0078] The heat treatment can be carried out in a vacuum or in hydrogenand/or argon as inert gases.

[0079] In addition to the metal component 13, the carrier substance andthe activation agent, the cleaning agent 10 may also have, for example,an organic binding agent (carboxylmethacrylate, carboxylmethylcelluloseor similar compounds), so that the cleaning agent 10 has a pasty orfoam-like consistency which can thus be applied well to the corrodedpart 1 and, by virtue of the binding agent, can adhere to the part 1,16.

[0080] A liquid also allows a cleaning agent compound which can bepoured to be produced, in which the part 1 is immersed, with thecleaning agent 10 adhering to the surface 7 of the part 1 once theliquid has dried.

[0081] The invention is not restricted to the application methodsmentioned.

[0082] Once the part 1 has been heat-treated for a specific time withthe cleaning agent 10, the concentration of the metal component 13 inthe area of the cleaning agent 10 facing the surface 7 is reduced. Onlya small amount of a metal component 13, or, in the extreme, no moremetal component 13, can diffuse into the part 1 from this area. Further,desired deeper penetration of the metal component 13 into the depth ofthe material 1 takes place only by further diffusion of the metalcomponent 13 which has already diffused into it. However, keeping thepart 1 at a raised temperature for a lengthy period would lead to themetal component 13 passing from a surface 11 of the cleaning agent 10via the gaseous compound to surface areas 8 of the part 1 to which nocleaning agent 10 has been applied, and when no penetration of themetallic component 13 or of the reaction products is desirable, either.

[0083] The cleaning agent is thus in this case removed from the heattreatment after a certain time, and only further, desirable penetrationof the metal component 13 into the depth of the material 1 takes placeby diffusion of the metallic component 13 which has already diffusedinto the part 1, on the basis of a thermal treatment of the part 1,without any cleaning agent 10. The thermal treatment is made possible,for example, by solution annealing of the part 1.

[0084] The removal of the cleaning agent 1 presents no problems sincethe metallic component 13 has not melted.

[0085] The cleaning agent 10 can be applied locally, in particular overthe areas which are more resistant to removal, over a large area or overthe entire area of the part 1, 16.

PARAMETER EXAMPLE

[0086] Layer Material: MCrAlY,

[0087] Depth of the corrosion products in the layer: 150 μm (depleted Alarea),

[0088] Application of the cleaning agent 10 results in a sacrificialzone 25 down to a depth of 80 μm during heat treatment at 925° C. for atime of two hours,

[0089] After removal of the cleaning agent, a thermal treatment iscarried out at 1120° C. for at most 20 hours:

[0090] The depth of the sacrificial zone 25 is 150 μm.

[0091] The duration of the thermal treatment and the temperature can beadapted on the basis of calibration curves (diffusion depth as afunction of the time and temperature) for the physical extent of thecorrosion products in the component.

[0092] A mask layer can be applied after the application of the cleaningagent 10 and before the heating process, in order to prevent themetallic component 13 from passing from the surface 11 of the cleaningagent 10 to surfaces 8 of the part 1 to which no cleaning agent wasapplied and where no penetration of the metallic component 13 isdesirable either. The cleaning agent 10 can thus remain on the part 1,with heat treatment nevertheless being carried out in order to achievethe effect described above.

[0093] The invention is not restricted to parts of gas turbines, butalso works in the case of parts which have at least one layer, forexample an oxidation protection layer, acid protection layer orcorrosion protection layer.

[0094] The invention is likewise not restricted to parts which have nolayers, but whose corrosion products must be removed, for example in thecase of reaction vessels in the chemical industry.

[0095]FIG. 7 shows a layer system 16 which comprises a substrate 19, forexample a nickel-based superalloy, an intermediate layer, in particularan MCrAlY layer 28, and an outer heat insulation layer 31.

[0096] The layer system 16 has been subjected to mechanical and thermalloads in use and is intended to be refurbished for use once again. Inthe process, the heat insulation layer 31 is removed, for example bysandblasting. This may be achieved easily by mechanical means, since theheat insulation layers 31 are generally ceramic, that is to say brittle,layers. The at least one intermediate layer 28 is metallic, and is moredifficult to remove by mechanical means.

[0097]FIG. 8 shows the layer system 16 from which the heat insulationlayer 31 has already been removed, and with the intermediate layer 28shown enlarged. The intermediate layer 28 is degraded. In a situationwhere corrosion products, that is to say oxides, nitrides and sulfides,have been formed or where phase segregation has taken place, degradationmeans, for example, coagulation of aluminum phases 43 or a change to theconcentration structure as a result of diffusion. However, theintermediate layer 28 does not necessarily appear as follows: in a firstzone 34 to which the heat insulation layer 31 was applied there areouter corrosion products 4 and inner corrosion products 5, which areproduced by contact and reaction with a reactive medium.

[0098] In a second zone 37, which is adjacent to the first zone 34 inthe direction of the substrate 19, there are, for example, no corrosionproducts, although diffusion caused by thermal loading has resulted incoagulation of aluminum, aluminum phases or other elements.

[0099] The second zone 37 is adjacent to a third zone 40, which islocated between the substrate 19 and the second zone 37. In the thirdzone 40, the concentration of the intermediate layer 28 has changed fromits original composition owing to diffusion of elements into thesubstrate 19. By way of example, in the case of an MCrAlY intermediatelayer 28 and an Ni—Al superalloy as the substrate 19, this is aluminum,whose concentration is higher in the MCrAlY layer than in the substrate19, and which thus diffuses into the substrate owing to theconcentration difference. Thus, for example, the entire intermediatelayer 28 is degraded, and represents the layer area 52 to be removed.

[0100] However, it is also possible for only the first zone and thesecond zone 34, 37 to be degraded and for the third zone 40 not toexhibit any degradation phenomena whatsoever. Nevertheless, the thirdzone 40 can also partially be included in a sacrificial zone 25, and canbe removed, by impregnation with the impregnation agent 13.

[0101] The method according to the invention as described in FIGS. 1 to4 is used to remove the entire intermediate layer 28, by theimpregnation agent 13 diffusing into the entire intermediate layer 28 asfar as the substrate 19 (FIG. 9). The intermediate layer 28 is removedas already described further above.

[0102]FIG. 10 shows a substrate 19, for example a nickel-basedsuperalloy for a turbine blade, which has been degraded by use in adegraded area 46 close to the surface, which represents the layer area52 to be removed. The degraded area 46 has been produced, for example,by corrosion, by diffusion of elements into the substrate 19, or bydiffusion of elements out of the substrate 19 into layers or layer areasof the substrate located on it.

[0103] The method according to the invention is used to introduce animpregnation agent 13 into the degraded area 46, so that the degradedarea 46 becomes a sacrificial zone 25, which can be removed completelyand more easily (FIG. 11). The layer 52 to be removed comprises at leastthe degraded area, but may also be larger than this.

[0104] The layers which can be removed by the method need notnecessarily be degraded. For example, FIG. 12 shows a layer system 16which comprises a substrate 19 and, for example, a chromium layer 49which has not been degraded and which represents the layer area 52 to beremoved, since a layer containing chromium or a chromium layer 49 ishighly resistant to removal by means of chemical removal methods.

[0105] However, the application example is not restricted to a chromiumlayer, and the chromium layer may also be degraded, for example bycorrosion. The layer 49 is difficult to remove by the normal removalmethods such as acid stripping.

[0106] The method according to the invention allows the impregnationagent 13 to penetrate into the layer 49, as a result of which the layer49 can be removed more easily by conventional methods, for example acidstripping (FIG. 13), since the resistance to removal has been reduced.

[0107] If the substrate 19 is likewise partially degraded, the heattreatment allows the impregnation component 13 to penetrate into thesubstrate, or the sacrificial zone 25 is enlarged by an extension zone54 as a result of diffusion during the thermal treatment.

1. A method for removal of at least one layer area of a corroded partcomposed of a metal and/or of at least one metal compound, comprising:locally applying a multicomponent cleaning agent to one surface of thecorroded part, the cleaning agent having an impregnation component thatcan diffuse into the layer area, and the cleaning agent having anactivation component; heat treating of the part with the cleaning agentso that the at least one impregnation component and the activationcomponent form gaseous compound; forming of at least one sacrificialzone at least partially in the layer area which is to be removed fromthe part by the heat treating, and by the gaseous compound coming intocontact with the part, as a result of which a removal resistance of thelayer areas is reduced; and removing the layer with the sacrificialzone.
 2. The method as claimed in claim 1, wherein the cleaning agent atleast partially adheres to the part.
 3. The method as claimed in claim1, wherein the at least one sacrificial zone is at least partiallyformed by areas of the part which have the at least one impregnationcomponent.
 4. The method as claimed in claim 1, wherein the cleaningagent has, as at least one impregnation component, at least one metalcomponent composed of a metal or of a metal alloy, or a component whichcontains metal.
 5. The method as claimed in claim 1, wherein the atleast one impregnation component penetrates by diffusion into the part,directly from the gas phase or after deposition on the part.
 6. Themethod as claimed in claim 1, wherein the temperature during the heattreatment of the part with the cleaning agent is below the lowestmelting point of the at least one impregnation component.
 7. The methodas claimed in claim 1, wherein the cleaning agent contains a halogencompound as at least one activation component.
 8. The method as claimedin claim 4, wherein the metal component is composed of aluminum, or thecomponent which contains metal contains aluminum.
 9. The method asclaimed in claim 1 wherein the at least one sacrificial zone has, atleast partially, aluminum or aluminum compounds.
 10. The method asclaimed in claim 1, wherein the cleaning agent has at least one carriersubstance as a further component.
 11. The method as claimed in claim 10,wherein the carrier substance is aluminum oxide.
 12. The method asclaimed in claim 1 wherein the cleaning agent has a pasty consistency.13. The method as claimed in claim 1 wherein the cleaning agent containsat least one binding agent, in order to produce the pasty consistency ofthe cleaning agent, as a further component.
 14. The method as claimed inclaim 1 wherein the part is a layer system having at least one layer, inparticular a coated turbine blade, with the layer area which is to beremoved corresponding to the layer.
 15. The method as claimed in claim14, wherein the layer is a MCrAlY layer.
 16. The method as claimed inclaim 1 wherein external corrosion products in the surface of the partare removed.
 17. The method as claimed in claim 1 wherein internalcorrosion products underneath the surface of the part are removed. 18.The method as claimed in claim 1 wherein the cleaning agent is removedin an intermediate step after the heat treatment.
 19. The method asclaimed in claim 18, wherein the at least on sacrificial zone in thedepth of the part is enlarged by means of a thermal treatment in anintermediate step.
 20. The method as claimed in claim 19, wherein thetemperature of the thermal treatment is at least partially above thetemperature of the heat treatment.
 21. The method as claimed in claim 4wherein the temperature of the thermal treatment allows solutionannealing of the part.
 22. The method as claimed in claim 1 wherein thecleaning agent is applied to the surface of the part in the area ofcorrosion products.
 23. The method as claimed in claim 1 wherein thegaseous compound produces an impregnation layer in the part theimpregnation layer composed at least partially of the at least oneimpregnation component.
 24. The method as claimed in claim 1, whereinthe layer area contains corrosion products.
 25. The method as claimed inclaim 1, wherein the layer area is degraded.
 26. The method as claimedin claim 1, wherein the layer area is degrade, by diffusion of chemicalelements from or into the layer area.
 27. The method as claimed in claim1, wherein the layer area is a chromium layer or a layer which containschromium.